The present invention generally relates to recording and reproducing apparatus and, more particularly, to apparatus that is adapted to record and reproduce television signals, using digital techniques.
The continued advances in technology have resulted in many changes in the equipment that is currently being used in television broadcast stations. One of the more recent changes that has evolved is the shift away from photographic techniques toward the use of magnetic media in many phases of the operation of the commercial broadcast television station. For example, feature films being broadcast often originate from magnetic tape rather than film and television station news departments are increasingly converting to videotape recording systems rather than using film cameras to provide the visual coverage of the news stories. Moreover, many systems utilize travelling transmitters that can either broadcast on location coverage or transmit such coverage to the station which can either be broadcast "live" or videotaped, edited and broadcast at a later time. Some of the many benefits of these techniques are the ease of handling, flexibility and speed of processing compared to the use of photographic film, coupled with the ability to reuse the magnetic tape when the information that is recorded on them is no longer needed.
One of the last remaining film domains in the present day commercial television broadcasting station is the Telecine island which uses 35 millimeter film transparencies. The Telecine island is used to provide video still images that are used during programming, commercials, news and the like, i.e., wherever a still image may be used during operation. Their use is extensive as is evidenced by the fact that the average commercial broadcast television station maintains a total file on the order of about 2000 to 5000 35 millimeter transparency slides. The maintenance of the total file represents a laborious operation which requires introduction of new slides, the discarding of obsolete slides and the maintenance of an accurate index so that they can be readily obtained when needed. When slide program sequences are to be assembled, they must be manually carried to the Telecine island, cleaned and manually loaded. Even with the cleaning operation, dust particles and scratches and the like may easily result in an unsatisfactory end product even when the projectionist is careful. Morever, following their use during broadcasting, the slides must be removed and returned to the file. The entire assembling, use and refiling of the slides represent a substantial labor investment because of the many manual operations that are required. The Telecine operation is considered to be one of the most antiquated operations in many modern broadcast stations and is basically incompatible with a fully automated station operation.
In contrast to the Telecine island or the use of opaque graphic material as the source for generating video still images, the apparatus described herein facilitates the use of a recording and playback apparatus that will record and reproduce still images, with the still image video information being stored on magnetic media. The magnetic recording and playback apparatus utilizes generally standard computer disc drives (though modified in some respects) as the magnetic storage media and thereby eliminates the many problems that are associated with slide transparencies. Since the still images are recorded on magnetic media, the problems of physical degradation during use, e.g., dust particles and scratches, are not experienced. Moreover, since the recorded information can be easily accessed, the same still image may be used by operators at different locations almost simultaneously.
As will be hereinafter described in detail, in the present invention the video information signal is sampled at a rate that is a multiple of the subcarrier frequency so that a number of samples are taken during each cycle of subcarrier. In the illustrated embodiment, the apparatus which is controlled by a sampling clock samples the video information signal at a rate of three times the subcarrier frequency so that three samples are taken for each cycle of subcarrier, i.e., at the 0.degree., 120.degree. and 240.degree. phase locations or positions. Since for a color video information signal, the phase of the samples relative to subcarrier determines the hue of the video image upon reproducing, it should be understood that sampling the information signal at other than the precise phase positions will ultimately produce inferior color reproduction.
The apparatus of the present invention insures that the samples are taken precisely at the phase locations that are desired so that the color will be accurate upon subsequent reproducing. The apparatus utilizes a phase locked loop to provide sampling reasonably close to the preferred sample locations, i.e., within about 20.degree. of each of the desired sample locations of 0.degree., 120.degree. and 240.degree. in the illustrated embodiment. The actual samples are then examined to determine if phase error does in fact exist and adjusts the sampling clock so that the samples are taken precisely at the 0.degree., 120.degree. and 240.degree. phase positions. This effectively corrects any errors that may be produced due to temperature drifting and the like.